Water deficit is one of the major limiting factors for plant productivity and plant distribution (Boyer 1982). Plants respond to drought by modifying their morphological, physiological and metabolic processes (McCue and Hanson 1990). Depending on the efficiency of the response, plant species can withstand longer or shorter periods of water deficit. Some responses may result from cell damage while others may correspond to adaptive processes (Bray 1997). Plant adaptive responses to water stress are accompanied by the accumulation of specific mRNAs (Shinozaki and Yamaguchi-Shinozaki 1997). In addition, the mRNAs of water-stress-inducible genes decrease when the plants are released from stress conditions, which is consistent with evidence that shows that these genes respond to water stress. It is believed that the altered phenotype of plants with an enhanced ability to survive and grow under environmental stresses is largely the result of changes in gene expression (Ingram and Bartels 1996; Shinozaki and Yamaguchi-Shinozaki 1997). Most of the genes that respond to drought are also induced by exogenous application of abscisic acid (ABA) (Merlot and Giraudat 1997; Bray et al. 1997). It has been well documented that ABA has important roles in the tolerance of plants to drought. For example, applied ABA inhibit stomatal opening and promotes stomatal closure (Blatt and Thiel 1993; Ward et al. 1995). It appears that water stress triggers the production of ABA, which in turn induces various genes. A number of genes that respond to drought and exogenous ABA at the transcriptional level have been recently isolated and characterized from several species (for review, see Ingram and Bartels 1996; Bray 1997; Shinozaki and Yamaguchi-Shinozaki 1997). The functions of these gene products are thought to have a role in protecting the cells from water deficit (Ingram and Bartels 1996; Bray 1997).
The homeobox genes are characterized by a conserved 180-bp nucleotide sequence known as the homeobox, which encodes a 60-amino acid DNA binding homeodomain (HD) structured in three .alpha.-helices (Gehring et al. 1994). This DNA binding property indicates that homeodomain proteins function as transcription factors in controlling downstream target genes. The HD factors controls several developmental decisions in animals, apparently acting as molecular switches to control the fates of the cells during development (reviewed in McGinnis and Krumlauf 1992; De Robertis 1994). In higher plants, a class of the HD genes was first discovered in Arabidopsis thaliana (Ruberti et al. 1991). Unlike other classic homeobox proteins, the products of these genes contain a second element that codes for a putative leucine zipper motif, which is closely linked to the carboxy-terminal region of the HD. So far, these termed homeodomain-leucine zipper (HD-Zip) proteins have been identified only in plants such as sunflower (Chan et al. 1994), carrot (Kawahara et al. 1995), soybean (Moon et al. 1996), tomato (Meissner and Theres 1995), rice (Meijer et al. 19970 and Arabidopsis (Mattsson et al. 1992; Soderman et al. 1994; Lee and Chun 1998). The uniqueness of the HD-Zip proteins in plants suggests that these HD-Zip proteins function as a mediator of plant development; for example, coupling of the developmental response to an environmental signal (Schena and Davis 1992). Several lines of evidence support this notion.
We have recently isolated a novel homeobox-containing gene, Athb-12, which was induced by water deficit and exogenous ABA treatment (Lee and Chun 1998). The yeast genetic study revealed that the Athb-12 protein contains a transcriptional activation domain in the C-terminal region. In addition, this protein was found to bind to the 12 bp palindromic sequence EN (TCAATTAATTGA), which is the consensus recognition site determined for Drosophila Engrailed and a number of other animal HD proteins (Desplan et al. 1988). These data indicate that the Athb-12 is a transcription factor involved in the regulation of the plant's response to water deficit. Generally, transcription factors regulate multiple target genes. Thus, the overexpression of Athb-12 in plants may increase the resistance to drought. Recently, increased expression of Arabidopsis CBF1, a transcription factor that binds to the CRT/DRE sequence, induced COR (cold-regulated) genes and increased the freezing tolerance of Arabidopsis plants (Jaglo-Ottosen et al. 1998).